This invention relates to thermoplastic elastomer compositions comprising blends of graft copolymers, polyvinyl chloride (PVC) and plasticizers. The compositions require no curing or vulcanization to develop elastomeric properties. The compositions thus remain thermoplastic and can be repeatedly remolded or extruded.
The term "thermoplastic elastomer" has generally been applied to elastomers that can be readily processed and reprocessed by conventional melt processing equipment by virtue of the fact that such elastomers are not cured or vulcanized. The reprocessability of these elastomers compared with conventional cured or thermoset rubbers results in a great reduction in loss due to scrap, with consequent economic benefits for the processor. A variety of such materials have been introduced in recent years such as thermoplastic polyesters, styrene block copolymers, and thermoplastic olefin-rubber blends. Typical of such materials are the styrene-butadiene-styrene block copolymers sold as Kraton brand elastomers by the Shell Chemicals Co. and the Hytrel brand polyester elastomers sold by DuPont. Many of these elastomers have found wide application in consumer goods such as in shoe soling formulations and the like, as well as in such industrial applications as wire coating, hose and tubing, electrical connectors and automotive parts.
Currently available thermoplastic elastomers suffer some disadvantages in use. In particular, formulations based on olefinic resins including SBS block copolymers exhibit poor resistance to hydrocarbon solvents and low abrasion resistance which may limit their use in particular environments. Additionally, adhesion to dissimilar materials is poor, and a surface chlorination or other primer treatment is often needed to increase adhesive bonding between, for example, a molded shoe sole formed of such materials and a synthetic shoe upper. Primer treatment of the surfaces of molded goods is also needed where the part is to be painted, which further increases the production cost of such goods.
Nitrile rubbers or elastomers are essentially random, non-crystalline copolymers of 1,3-dienes and acrylonitrile containing from 15 to 50% acrylonitrile. These rubbers are widely available commercially and have long been used in the manufacture of oil-resistant gasketing, hoses and the like. As produced, nitrile rubbers are generally soft, low-strength thermoplastic gums that are soluble in or swelled by a variety of solvents. When compounded with reinforcing fillers and vulcanized, nitrile rubbers are tough useful elastomers with excellent oil and solvent resistance. The oil and fuel resistance of cured nitrile rubbers generally increases with increasing acrylonitrile level. However, the improved oil resistance is gained at some sacrifice in resilience and low temperature flexibility. Further, the vulcanizing or curing process results in highly cross-linked materials which are insoluble and intractable. The cured nitrile elastomers thus become thermoset and are no longer thermoplastic and readily reprocessable.
The modification of nitrile rubber stocks by adding PVC together with a conventional PVC plasticizer has long been practiced in the rubber compounding art. A minor proportion of PVC, usually less than 33 wt %, is used to impart increased sunlight and ozone resistance to nitrile rubber, together with improved abrasion and tear properties. Such formulations find use in wire and cable coverings and in the production of hose and tubing, as well as in shoe sole formulations. These blends are, for most applications, normally vulcanized to provide elastomeric character and therefore are not considered to be thermoplastic elastomers.
Oil-resistant thermoplastic elastomers comprising graft copolymers prepared by graft copolymerizing mixtures of monovinyl aromatic monomers and vinyl nitrile monomers in the presence of a nitrile rubber substrate have recently been disclosed. These compositions are thermoplastic elastomers without being vulcanized, and exhibit a high degree of oil resistance while retaining low temperature properties. For some applications, however, these materials exhibit an undesirably high level of shrinkage in molding and further formulation is required.
Thermoplastic elastomer formulations based on nitrile rubber graft copolymers would be a useful advance in the art. These graft copolymers are readily produced by a variety of well known and economical processes. As is well known, graft copolymers may be readily modified by varying the type and proportion of monomers used in their preparation to selectively improve such characteristics as abrasion and solvent resistance, adhesion, weatherability and the like. Elastomer formulations based on these graft copolyemrs could thus find application in the production of molded and extruded goods to meet a wide variety of environmental requirements including shoe soling, extruded hose and tubing, wire and cable insulations, the production of flexible cord, automotive parts and the like.